1. Field of the Invention
The present invention relates to a compact high-zoom-ratio real-image finder, and more specifically, to a compact high-zoom-ratio real-image finder suitable for use in a lens shutter camera.
2. Description of the Prior Art
In recent years, a lens shutter camera has been required to have a high zoom ratio and to be small in size. It is therefore natural that a finder incorporated in the lens shutter camera is required to have specifications and configuration which meet the requirements. For example, to make the camera compact by reducing the thickness thereof, it is essential to decrease the length of a finder objective lens system (hereinafter referred to as "objective system").
U.S. Pat. No. 4,842,395 discloses a real-image zoom finder where the objective system comprises from the object side a negative and a positive lens units, zooming is performed by moving the second lens unit having a positive power, and the variation in dioptric power caused by zooming is compensated for by moving the first lens unit having a negative power. Moreover, U.S. Pat. No. 5,028,125 discloses a real-image zoom finder where the objective system comprises from the object side a positive, a negative and a positive lens units, and zooming is performed by moving the second lens unit having a negative power. Further, Japanese Laid-open Patent Application No. H3-153206 discloses a real-image zoom finder wherein the objective system comprises from the object side a positive, a negative, a negative and a positive lens units, and zooming is performed by moving the second and third lens units having a negative power.
In the type of the finder as disclosed in U.S. Pat. No. 4,842,395, it is difficult to excellently correct aberrations while achieving a high zoom ratio, and further, since the movement amount of the second lens unit having a positive power largely increases as the zoom ratio increases, the total length of the objective system increases. As a result, the thickness of the camera cannot be reduced. That is, to achieve a high zoom ratio with the two-unit objective system of negative, positive configuration, it is necessary to increases the movement amount of the second lens unit having a positive power. To secure the space therefor, the camera should be thick.
The movement amount can be reduced by increasing the power of each lens unit. However, if the power of each lens unit is increased, the sensitivity to work errors is high. The sensitivity to work errors is a degree of influence by the decentering of the lens. That the sensitivity to work errors is high means, for example, that the dioptric power variation and the parallax are large when the lens is decentered by a predetermined amount. When the sensitivity to work errors is high, it is difficult to manufacture the finder within a permissible work error range. Specifically, since a certain degree of backlash is necessary in the attachment structure of the lens to move the lens, when zooming is performed by moving a lens highly sensitive to errors, the suppression of work errors so as to be within the permissible range is difficult compared to the case where the lens is stationary.
In the type of the finder as disclosed in U.S. Pat. No. 5,028,125, it is difficult to excellently correct aberrations while achieving a high zoom ratio, and further, if the zoom ratio is increased, the variation in dioptric power caused by zooming increases. For this reason, it is necessary to correct the dioptric power by moving the first lens unit having a positive power or the third lens unit having a positive power or both of the first and third lens units. However, in the three-unit objective system of positive, negative, positive configuration, since the positive power is strong in the first and third lens units, the sensitivity to work errors (in particular, the sensitivity to the shift of the field due to the decentering of the lens) is high. Therefore, similarly to the above-described finder as disclosed in U.S. Pat. No. 4,842,395, the suppression of work errors so as to be within the permissible range is difficult in this type of finder.
The type of the finder as disclosed in Japanese Laid-open Patent Application No. H3-153206 employs a four-unit objective system of positive, negative, negative, positive configuration with which it is easier to correct aberrations than with the other types. Further, since the first and fourth lens units having a strong positive power (the power of the fourth unit is particularly strong) are stationary, work errors are suppressed more easily. However, since the image formation magnification of the second lens unit having a negative power is negative in the entire zoom range, the third lens unit moves so as to make a U-turn. If the third lens unit makes a U-turn, the balance between the absolute movement amounts of the second and third lens units is broken, so that although the absolute movement amount of the third lens unit decreases, the movement amount of the second lens unit increases. Since the total length of the four-unit objective system of positive, negative, negative, positive configuration depends on the lens unit which largely moves, the total length of the objective increases in accordance with the increase in movement amount of the second lens unit. Therefore, in this type of finder, it is difficult to reduce the thickness of the camera.